This invention relates to a method of producing surface-mountable thermistor devices which may be used for protection against an overcurrent. More particularly, this invention relates to a method of producing organic thermistor devices comprising a thermistor element made of an organic thermistor material.
Organic PTC (positive temperature coefficient) thermistors made of an organic thermistor material are coming to be used as circuit protection units for suppressing overcurrents. Such organic PTC thermistor devices make use of an organic thermistor material obtained by dispersing carbon or the like in a resin material such as polyethylene to provide a positive temperature characteristic (PTC characteristic). They are generally produced, as shown in FIG. 6, by forming surface electrodes 52a and 52b by pressing a metallic foil of nickel or copper on both upper and lower surfaces of a thermistor body 51 of an organic thermistor material shaped in a planar form and then forming outer electrodes 53a and 53b by plating or sputtering. Alternatively, an organic thermistor device may be formed, as shown in FIG. 7, by using an electrically insulating material 54 such as an insulating resin to cover exposed parts such as the thermistor body 51 and the surface electrodes 52a and 52b, leaving only the outer electrodes 53a and 53b exposed.
An organic thermistor device, as described above, may be surface-mounted, as shown in FIG. 8, by electrically and mechanically connecting the outer electrodes 53a and 53b to wiring electrodes (or xe2x80x9clandsxe2x80x9d) 56 on a printed circuit board 55 by a solder reflowing method through a solder fillet 57.
In the case of a PTC thermistor device for protecting a circuit from an overcurrent situation, its resistance value at normal temperatures is desired to be 0.1 xcexa9 or less such that a voltage drop in the PTC thermistor device during the use of the circuit can be avoided. If the specific resistance, the thickness and the cross-sectional area of the PTC thermistor body 51 are xcfx81, T and S, respectively, the resistance value of the PTC thermistor device is given by xcfx81T/S.
If an organic PTC material is to be used for the PTC thermistor device, the fact is that it is currently considered difficult to make the specific resistance equal to or less than 0.5 cm if this PTC thermistor material must also have the required electrical characteristics when its resistance value changes suddenly under a high-temperature condition. Accordingly, if it is attempted to use such an organic PTC thermistor material to produce an organic PTC thermistor device with resistance value equal to or less than 0.1 xcexa9 at normal temperatures, the result will be a structure as shown in FIG. 7 having surface electrodes 52a and 52b formed on both upper and lower surfaces of a planar thermistor body 51 made of an organic thermistor material by pressing a metallic foil of nickel or copper.
Even if a PTC thermistor device is produced in a form as shown in FIG. 7 with surface electrodes on both upper and lower surfaces of the thermistor body, the thickness of the thermistor body 51 must be made very small and its cross-sectional area large in order to make its resistance value at normal temperatures equal to or less than 0.1 xcexa9. With prior art organic PTC thermistor devices, therefore, the dimensions of the thermistor body 51 were, for example, 4.5 mm (length)xc3x973.2 mm (width)xc3x970.3 mm (thickness).
Although it is an essential requirement for a PTC thermistor device to have a reduced resistance value at normal temperatures, this requirement could be satisfied with the prior art technology only by reducing the thickness of the thermistor body and increasing its cross-sectional area (or its planar area). As a result, the planar dimensions of the product remained large and a large space was required for its surface-mounting. Secondly, a relatively large amount of organic thermistor material will be used for the production and this gives rise to an increased production cost. Thirdly, if the thermistor body is very thin, it is likely to become twisted or bent after being mounted. Fourthly, if a large amount of the organic thermistor material is used between the pair of outer electrodes, the action time of the PTC thermistor device becomes long and there may arise situations where a sufficient protective characteristic against overcurrents cannot be obtained and the circuit element to be protected may break before the PTC thermistor device can act.
An attempt may be made to introduce inner electrodes into the PTC thermistor body by stacking organic PTC sheets with an electrode formed thereon, but the layer-forming process including steps of making thinner organic PTC sheets, forming conductors to serve as inner electrodes and stacking up the sheets one on top of another tends to increase the production cost as a whole. Thus, the price of the product will increase significantly and hence such a method is not a practical solution to the problem.
It is therefore an object of this invention, in view of the problems described above, to provide a method of producing compact organic thermistor devices which have a small resistance value at normal temperatures and are economically advantageous.
Organic thermistor devices to be produced according to this invention may each be characterized as comprising a thermistor body made of an organic thermistor material, a pair of outer electrodes on mutually opposite end parts of the thermistor body and facing each other, and a plurality of mutually parallel longitudinally extending planar inner conductors with thickness 10-200 xcexcm disposed inside the thermistor body. Each mutually adjacent pair of these inner conductors are connected to different ones of the outer electrodes and has main surfaces which are in a face-to-face relationship with each other with the organic thermistor material inserted in between. The externally exposed surfaces of the device, except where the outer electrodes are formed, may be covered by an insulating material for preventing unwanted electrical contact of the thermistor body or the inner conductors with other conductors such as various components and wires on a circuit board.
Alternatively, these planar conductors may be replaced by a plurality of metallic wires, or a bar with a circular or quadrangular cross-sectional shape. Since the specific resistance of the metallic conductor is negligibly small, compared to that of the organic thermistor material, the resistance value of the device can be thereby reduced.
A method according to this invention for producing such organic thermistor devices may be characterized by the steps of molding an organic thermistor material by covering a plurality of electrically conductive plates to thereby form an elongated conductor-containing member having these conductive plates buried parallel to one another inside the organic thermistor material such that each mutually adjacent pair of these conductive plates is externally exposed on different ones of mutually oppositely facing side surfaces of the member, forming a pair of longitudinally elongated electrodes on these side surfaces, and thereafter cutting this conductor-containing member transversely at specified positions to thereby divide into individual units. Such electrodes may be formed by coating externally exposed surfaces of the conductor-containing member entirely with an electrically insulating material, thereafter removing portions of it from the side surfaces to thereby expose edges of the conductive plates, and thereafter forming the electrodes on the side surfaces.
By initially forming such a conductor-containing member, organic thermistor devices of this invention can be produced efficiently.